These days more and more condominium and apartment balconies are cracking and crumbling – resulting in balcony repair and restoration. This is a safety concern, both for the residents and for pedestrians below, and it is also virtually unattractive.
Whether obvious or hidden, this type of deterioration is occurring in a great many reinforced concrete balcony slabs. It is caused by galvanic corrosion of reinforcing steel due to high concentrations of salt in the concrete and is frequently compounded by an inadequate depth of concrete covering the reinforcing steel. It is normally first visible as cracking, spalling and delamination of concrete at slab edges.
The salt present in a great number of concrete balconies finds its way into the slabs in one, or more, of four (4) ways:
- In the aggregate at the time of mixing,
- Added to the concrete at the time of mixing to make it gain strength more quickly,
- Placed on the top surface of the concrete during periods of cold weather to remove, or prevent ice buildup,
- Deposited on the surface of the concrete during periods of cold weather to remove or prevent, ice build-up, and
- Deposited on the surface of the concrete during wintertime in the spray thrown up from the streets below by passing traffic and then carried to the balconies by the wind.
Salt in the Aggregate
It has been established by the Ministry of Transportation that some of the limestone deposits in the Niagara Peninsula quarried as a source of concrete aggregate contain salt concentrations that will create a chloride level in the hardened concrete that is approximately 67% of the threshold concentration for galvanic corrosion of the reinforcing steel. This salt is present only in the aggregate, not the cement paste component of the concrete, and as such does not contribute to the rusting problem.
Salt Added to the Concrete
At the time a great many apartment buildings were built, it was a general custom in the ready-mixed concrete industry in the wintertime to add 1.5% calcium chloride by weight of cement to the concrete as an admixture to combat the effects of the cold weather on the concrete and to reduce the time taken for the concrete to harden. This practice resulted in the concrete placed in the wintertime frequently containing concentration of salt well above the level required for galvanic corrosion of the reinforcing steel to occur.
Salt Placed on the Balconies by Occupants
Occasionally unit owners will keep their balconies free ice and snow by spreading de-icing salt on the slab. This will cause concrete deterioration in a way similar to that in parking garages.
Salt from the Roads
During investigations carried out to determine the present condition of balcony slabs and reinforced concrete balcony enclosures, it has been found that a correlation exists between the concentration of salt present in the concrete forming a balcony slab and it’s position on the building in relation to adjacent street and it’s degree of exposure to wind borne salt laden spray originating on the street.
There are a great many factors that significantly affect the salt concentration in a balcony slab due to this source of salt. They include the following:
- Type of street and traffic volume,
- Height of the balcony, distance from the street and whether it faces the street,
- Quality of the concrete forming the balcony slab as it affects the permeability of the concrete and the volume of permeable pores present in the concrete, and
- Degree to which the slab is protected by the balcony enclosure.
- The drainage of the balcony slab,
- Presence of a protective membrane or sealer on the top surface of the concrete, and
- Presence of outdoor carpeting on the balcony slab.
Consequences
The cracking and delamination of slab edge concrete brought about by galvanic corrosion frequently creates a dangerous situation with a potential for pieces of concrete to fall and possible injure pedestrian. It also poses and delamination of slab edge concrete brought about by galvanic corrosion frequently creates a dangerous situation with a potential for pieces of concrete to fall and possibly injure a pedestrian.
It also poses risks for people on the balconies. In a cantilvered balcony slab, a crack may appear in the top surface of the concrete parallel to the building and immediately adjacent to the wall. Such a crack may be indicative of excessive deflection and it is possible that the balcony may collapse when subjected to a sudden increase in load at one time such as a large number of people assembling on the balcony.
Balcony slabs normally form an integral part of the floor slab within the building and the concrete forming balcony slabs is therefore of the same quality as the concrete forming the floor slab. Thus a minor problem that is often present in the balcony slabs, unrelated to salt, the absence of entrained air from the concrete and the related susceptibility of the surface of the concrete to scaling as a result of being subjected to cycles of freezing and thawing while in saturated condition. The potential for surface deterioration of this type to occur is increased both by poor drainage and by outdoor carpeting on the slab.
While scaling in unlikely to affect the performance of a balcony slab structurally, it is good reason for sealing the top surface of these slabs against water penetration at the time of construction.
Prevention and Deflection
If balcony slabs are sealed this provides a good measure of protection against water and salt penetration. However, most balcony slabs are initially unsealed. Since it is likely that salt has already built up in the slab, later sealing may be of little use in preventing structural damage.
Galvanic corrosion of the reinforcing steel at the edges of the balcony is readily visible from ground level, and a survey with a pair of binoculars reveals all locations at which the condition has developed to a degree that action is necessary both to remove the safety hazard and to restore the slab to a visually attractive and sound condition.
Unfortunately, galvanic corrosion of the main reinforcing steel in the top of the balcony slabs, which could eventually result in both delamination of the concrete cover the reinforcing steel and in separation of the reinforcing steel from the underlying concrete, is not readily apparent to the casual observer.
Another source of concern which has not been discussed in this article is the progressive deterioration on concrete and metal balcony enclosures that is occurring to such an extent in some of our older building that the connections holding them in place are becoming potentially unsafe.
Conclusions
Salt is no longer added to the concrete at the time of mixing. Use of de-icing salt and carpeting on balconies is discouraged. However, road salt migration onto balconies is difficult to control, thus the only true defense against deterioration is awareness of the problem and of the factors that cause it. Periodic inspection and maintenance are the keys to prevention.
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